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Let x and y be real numbers such that

2<xyx+y<5.
If xy is an integer, what is its value?

 Jul 30, 2019
 #1
avatar
+2

See this link:  https://www.wolframalpha.com/input/?i=Give+integer+solutions+++++%5B++2+%3C+%5Cfrac%7Bx+-+y%7D%7Bx+%2B+y%7D+%3C+5+%5D

 Jul 30, 2019
 #2
avatar+9675 
+2

Let xy=z.2<xyx+y<52<z1z+1<52z+21<z12<5z+51:2z+2<z1z<32:z1<5z+56<4zz>32Combine 1 and 2:32<z<3The only integer solution is z=2xy=2

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 Jul 30, 2019
 #3
avatar+130466 
+3

One question, Max

 

If  the solution is  -3/2 < z < -3

 

What integer  is > -3/2   and < -3  ???.....it can't be  z =   -2

 

 

cool cool cool

CPhill  Jul 30, 2019
 #4
avatar+33654 
+4

Note that x-y must be larger than x+y if the ratio is to be between 2 and 5. This will be the case if y is negative.

 

So we have (x-y)/(x+y) > 2,  or. x-y > 2x+2y or x < -3y or x/y > -3 where the < becomes > because we are dividing by a negative number.

 

We also have (x-y)/(x+y)<5,  or x-y < 5x+5y or 4x > -6y or x > -(3/2)y or x/y < -3/2 where > becomes < because we are dividing by a negative number.

 

Hence -3 < x/y < -3/2, giving x/y = -2

 

Max got the right answer with incorrect reasoning! It happens!

 Jul 30, 2019
 #5
avatar+130466 
+2

Thanks, Alan.....that makes sense.....!!!!

 

cool cool cool

CPhill  Jul 30, 2019
 #6
avatar+159 
+1

Thank you to the numerous amount of people who answered this question

 Jul 31, 2019

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